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3.6 a. High-temperature in-situ FTIR study of H2O-CO2 fluid inclusions (R.L. Linnen and H. Keppler, in collaboration with S.M. Sterner/Richland)

Every year several hundred papers are published that make use of fluid inclusion data, most of which concern aqueous-carbonic fluids, yet there is no accurate method of determining the H2O/CO2 ratios and hence the molar volumes of these inclusions. Consequently large errors are associated with the interpretation of fluid pressure, temperature and composition A new technique has therefore been developed to analyse fluid inclusions in the one-phase field (above the homogenization temperature) by FTIR spectroscopy. The technique consists of measuring the peak area ratios of H2O and CO2 in synthetic H2O-CO2 fluid inclusions of known density and composition at a temperature in the one phase field (Fig. 3.6-1) and thereby determining the relative extinction coefficients for H2O and CO2. A 'calibration curve' can then be used for the analysis of inclusions with unknown composition. A standard gas-flow fluid inclusion stage has been modified for use with the FTIR spectrometer by replacing the glass plates in the sample chamber with CaF2 windows. The quartz sample containing the fluid inclusions rests on an inconel plate with a triangular hole and is held in place by a thermocouple. Calibration tests with critical water inclusions show that temperature gradients are negligible and the inclusion temperature is within 2 °C of the thermocouple temperature. IR measurements were carried out using a Bruker infrared microscope which allows focusing of the infrared beam down to an inclusion size of less than 10 µm.

Fig. 3.6-1: Representative infrared spectra of synthetic H2O-CO2 fluid inclusions with 12.5 and 25 mole % CO2 measured in-situ at 300 °C.

The results in Fig. 3.6-2 were all obtained at 300 °C and the H2O/CO2 peak area ratios show a continuous variation with fluid composition. Density also has a weak effect on the extinction coefficient ratio but, by combining microthermometric and spectroscopic data it is possible to simultaneously solve for the density and composition of an "unknown" a fluid inclusion. The effect of additional components, such as NaCl, KCl and CH4, on the infrared spectra and relative extinction coefficients is currently being investigated. These experimental calibrations will allow a much more precise determination of paleo-fluid compositions and trapping conditions than is presently achievable. The technique developed here is not only important as a new analytical tool for fluid inclusions but can also be applied to high temperature spectroscopy in general, using synthetic fluid inclusions instead of classic cells, where the control of molar volume and composition is more difficult, and where corrosion problems at high temperature with some fluid compositions may be encountered.

Fig. 3.6-2: Calibration curve of H2O/CO2 peak area ratio at 300 °C as a function of fluid composition and molar volume.

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